Squeeze out the juice of the lemon and orange. Chop the garlic into round slices.
Salt the lamb well. Make holes or cuts with a knife on top of it - stick the garlic slices in them. Put the lamb in a large oven dish, pour the lemon and orange juice over it. Sprinkle with thyme, black pepper and cumin to taste. Cover well with foil and leave it in the fridge overnight.
Take it out of the marinade, put it in an oven dish. Let the butter soften at room temperature, smear it over the lamb.
Add 2-3 cups water, some olive oil and cover with foil. Bake in the oven at 392°F (200 °C).